Slices off the passed proportion of items from BOTH ends of the passed array (i.e., with proportiontocut=0.1, slices ‘leftmost’ 10% AND ‘rightmost’ 10% of scores. You must pre-sort the array if you want “proper” trimming. Slices off LESS if proportion results in a non-integer slice index (i.e., conservatively slices off proportiontocut).
Returns: trimmed version of array a